Append to JSON in Python (Optimally due to RAM constraint)

Question

I'm trying to find the optimal way to append some data to a json file using Python. Basically what happens is I have about say 100 threads open storing data to an array. When they are done they send that to a json file using json.dump. However since this can take a few hours for the array to build up I end up running out of RAM eventually. So I'm trying to see what's the best way to use the least amount of RAM in this process. The following is what I have which consumes to much RAM.

        i               = 0
        twitter_data    = {}
        for null in range(0,1):
            while True:
                try:
                    for friends in Cursor(api.followers_ids,screen_name=self.ip).items():
                        twitter_data[i]                     = {}
                        twitter_data[i]['fu']               = self.ip
                        twitter_data[i]['su']               = friends
                        i = i + 1
                except tweepy.TweepError, e:
                    print "ERROR on " + str(self.ip) + " Reason: ", e
                    with open('C:/Twitter/errors.txt', mode='a') as a_file:
                        new_ii = "ERROR on " + str(self.ip) + " Reason: " + str(e) + "\n"
                        a_file.write(new_ii)
                break

        ## Save data

        with open('C:/Twitter/user_' + str(self.id) + '.json', mode='w') as f:
                json.dump(twitter_data, f, indent=2, encoding='utf-8')

Thanks

Answer 1

My take, building on the idea from Glenn's answer but serializing a big dict as requested by the OP and using the more pythonic enumerate instead of manually incrementing i (errors can be taken into account by keeping a separate count for them and subtracting it from i before wriring to f):

with open('C:/Twitter/user_' + str(self.id) + '.json', mode='w') as f:
   f.write('{')
   for i, friends in enumerate(Cursor(api.followers_ids,screen_name=self.ip).items()):
        if i>0:
            f.write(", ")
        f.write("%s:%s" % (json.dumps(i), json.dumps(dict(fu=self.ip, su=friends))))
   f.write("}")

Answer 2

Output the individual items as an array as they're created, creating the JSON formatting for the array around it manually. JSON is a simple format, so this is trivial to do.

Here's a simple example that prints out a JSON array, without having to hold the entire contents in memory; only a single element in the array needs to be stored at once.

def get_item():
    return { "a": 5, "b": 10 }

def get_array():
    results = []
    yield "["
    for x in xrange(5):
        if x > 0:
            yield ","
        yield json.dumps(get_item())
    yield "]"

if __name__ == "__main__":
    for s in get_array():
        sys.stdout.write(s)
    sys.stdout.write("\n")